1. Field of the Invention
This invention relates to a metal mold device for molding a substrate of a disc as a recording medium for information signals, such as an optical disc or a magneto-optical disc. More particularly, it relates to a metal mold device for molding a disc substrate by molding synthetic resin, such as polycarbonate resin, and a method for molding a disc substrate by means of the metal mold device.
2. Description of Related Art
An optical disc has hitherto been proposed which makes use of a disc substrate formed of synthetic resin such as light-transmitting polycarbonate resin. This type of the optical disc may be exemplified by a read-only type disc and a magneto-optical disc with which it is possible to re-write previously recorded information signals. The read-only optical disc includes a disc substrate having its major surface formed with a pattern of recesses and lands corresponding to information signals, such as predetermined musical sound signals, or pits, and a reflective film produced by e.g. vacuum deposition of aluminum or the like material. The overwrite type magneto-optical disc, capable of re-writing information signals, has a signal recording layer having a magnetic film on its major surface on which a pre-groove constituting a recording track for recording predetermined information signals is formed.
The disc substrate, formed of synthetic resin for constituting the optical disc or the magneto-optical disc as described above, is produced using a metal mold device for injection tooling.
The metal mold device for molding the disc substrate is constituted as shown in FIG. 1. The metal mold device is made up of a fixed metal mold 1 and a movable metal mold 2 placed facing the fixed metal mold. A mold cavity 4 corresponding in shape to the disc substrate 30 to be molded is defined between the fixed mold 1 and the movable mold 2 making up a metal mold 3.
At a mid position of the fixed metal mold 1, there is provided a sprue busing 5 for introducing synthetic resin, such as molten polycarbonate resin, from a injection molding machine into the mold cavity 4. A resin injection port 6 is bored through the sprue bushing 5. The molten synthetic resin, supplied from the injection molding machine, is injected into the mold cavity 4 through the resin injection port 6.
A stamper 7 for molding the pattern of recesses and lands corresponding to the information signals, or pits, and a pre-groove constituting a recording track(s), is loaded on a surface of the fixed mold 1 defining the mold cavity 4. The stamper 7 is in the form of a disk having a central aperture 7a and is attached to the fixed metal mold by having the rim of the central aperture 7a supported by an inner stamper holder 8 and by having its outer periphery supported by an outer stamper holder 9.
The inner stamper holder 8, supporting the rim of the central aperture 7a at an inner side of the stamper 7, is loaded on the fixed metal mold 1 by being fitted to the outer periphery of a stamper holder support 10 which is fitted to an outer side of the sprue bushing 5. An annular stamper supporting pawl 11 is formed at a radially outer end of the stamper holder 8 for supporting the rim of the central aperture 7a of the stamper 7.
The inner stamper holder 8 is pressed and supported against a lateral surface 13a of a flange 13 of the stamper holder support 10 by a stamper holder fixing unit 12 which is passed through the inside of the fixed metal support 1. The inner stamper holder fixing unit 12 is made up of a holder fixing lever 14 traversing the fixed metal mold 1, a threaded part 15 connected to the holder fixing lever 14 and adapted for reciprocating the holder fixing lever 14 in the directions shown by arrows A and B in FIG. 1, and a knob 16 for rotating the threaded part 15. A wedge-shaped engaging supporting part 17 having an inclined lateral side is formed at a distal end of the holder fixing lever 14. The engaging supporting part 17 is engaged in a wedge-shaped mating engaging recess 18 formed on an outer lateral side of the inner stamper holder 8.
In the above-described construction of the inner stamper holder fixing unit 12, the threaded part 15 is rotated by means of the knob 16 for introducing the holder fixing lever 14 in the direction of arrow A in FIG. 1 for progressively engaging the wedge-shaped engaging supporting part 17 in the mating wedge-shaped engaging recess 18 for causing the inner stamper holder 8 to bear against the flange 13 of the stamper holder support 10. The stamper 7, having its inner edge supported by the stamper-supporting pawl 11, is loaded in position by being thrust and supported against a stamper contact surface 1a of the fixed metal mold 1.
The fixed metal mold 1 is also provided with an inner stamper holder releasing unit 19 for releasing the pressure support of the stamper 7 relative to the fixed metal mold 1 by the inner stamper holder 8. Similarly to the above-described inner stamper fixing unit 12, the inner stamper holder releasing unit 19 is made up of a holder releasing lever 20 traversing the fixed metal mold 1, a threaded part 21 connected to the holder releasing lever 20 and adapted for reciprocating the holder releasing lever 20 in the directions shown by arrows D and E in FIG. 1, and a knob 22 for rotating the threaded part 21. A wedge-shaped releasing part 23 having an inclined lateral side is formed at a distal end of the releasing lever 20. The releasing part 23 is engaged in a notch-shaped engaging surface 24 formed on an end face of the inner stamper holder 8.
For dismounting the inner stamper holder 8 fitted on the outer periphery of the stamper holder support 10 for detaching the stamper 7 from the fixed metal mold 1, the holder fixing lever 14 is receded in a direction shown by arrow B in FIG. 1 for disengaging the engaging supporting part 17 from the engaging recess 18. The threaded part 21 is then rotated by means of the knob 22 for advancing the holder releasing lever 20 in a direction shown by an arrow D in FIG. 1 for progressively engaging the releasing part 23 with the inclined surface 24 for moving the inner stamper holder 8 in the direction shown by arrow F in FIG. 1 for releasing the stamper from the stamper supporting pawl 11. The stamper 7 may now be detached from the fixed metal mold 1.
The movable metal mold 2, arranged facing the fixed metal mold 1 for being moved towards and away from the fixed metal mold 1, is provided with a sleeve 25 which is reciprocably mounted at the center of the mold cavity 4 for releasing a molded disc substrate 30. A punch 26 for punching a center hole 35 in the disc substrate 30 is reciprocably mounted at the center of the sleeve 25.
For molding the disc substrate 30, using the above-described metal mold device, the movable mold 2 is caused to approach the fixed metal mold for establishing a mold clamping state. In this mold clamping state, the molten synthetic resin, such as molten polycarbonate resin, supplied from the injection molding machine, is injected and charged into the mold cavity 4 via the resin injection port 6 of the sprue bushing 5. The movable mold 2 is caused to approach closer to the fixed metal mold 1 for compressing the synthetic resin charged into the mold cavity 4 under cooling for molding the disc substrate 30 corresponding in shape to the mold cavity 4. The punch 26 is then moved for boring the centering aperture. The movable mold 2 is moved away from the fixed mold 1 for mold opening, at the same time that the sleeve 25 is protruded towards the fixed metal mold 1 for releasing the molded disc substrate 30 out of the metal mold 3. The disc substrate 30 is taken out of the metal mold 3, using a suitable take-out unit, for forming the disc substrate 30 shown in FIG. 2.
In the above-described conventional metal mold device, since the stamper 7 is caused to bear against the fixed metal mold 1 by the inner stamper holder 8 provided with the stamper supporting pawl 11 adapted to support the rim of the central aperture 7a, the operation of attaching the stamper 7 to the fixed metal mold 1 is complicated. That is, for attaching the stamper 7 to the fixed metal mold, the inner stamper holder 8 is engaged in the stamper holder support 10 and the inner stamper holder fixing unit 12 is operated to shift the inner stamper holder 8 for thrusting the stamper 7 against the fixed metal mold 1 by the stamper supporting pawl 11, resulting in a complicated stamper attachment operation.
On the other hand, if the machining tolerance of the inner stamper holder 8 or the stamper holder support 10 to which the inner stamper holder 8 is fitted is not maintained within a narrow limit, the center of the stamper 7 may be offset with respect to the center of the disc substrate 30 to be molded. If the center of the stamper 7 is not in correct registration with the center of the disc substrate 30, the center of the pre-groove for forming the concentric or spiral recording track(s) or pits on the major surface of the disc substrate 30 by the stamper 7 is offset from the center of the disc substrate 30, so that the molded product cannot be used as a disc substrate for an optical disc.
Besides, the stamper 7 is caused to bear against the fixed metal mold 1 by the stamper supporting pawl 11 of the inner stamper holder 8, so that, if the thrusting force exerted by the stamper supporting pawl 11 is excessive, the stamper 7 tends to be distorted or deformed, so that rate of rejected products is likely to be increased.
In addition, the metal mold is complicated in structure by the provision of the inner stamper holder fixing unit 12 for fixing and supporting the inner stamper holder 8 or the inner stamper holder releasing unit 19 for detaching the inner stamper holder 8 from the fixed metal mold 1.
Above all, in the above-described arrangement in which the rim of the central aperture 7a of the stamper 7 is supported by the stamper supporting pawl 11 of the inner stamper holder 8 for attaching the stamper 7 to the fixed metal mold 1, the stamper supporting pawl 11 is protruded towards the disc substrate molding surface 7a of the stamper 7, so that the entire area of the molding surface 7a cannot be utilized effectively. That is, an annular groove 31 having a width corresponding to the stamper supporting pawl 11 is formed on an inner periphery of the disc substrate 30, as shown in FIG. 2, so that a signal recording region s in which a pre-groove for forming the pits or recording track(s) is formed cannot be extended to an area S.sub.1 at an inner most region of the disc substrate 30.
On the other hand, when the disc substrate 30 is formed as an optical disc, a reflective film or a signal recording layer 33 is deposited in the signal recording area s of the disc substrate 30 in which the pits or the pre-groove is formed, as shown in FIG. 3. Besides, a protective film 34 formed of a synthetic resin is deposited on the reflective film or the signal recording layer 33. The function of the protective film 34 is to prevent air or moisture from being contacted with and attacking the reflective film or the signal recording layer 33.
Consequently, the protective film 34 needs to be firmly deposited on the disc substrate 30 so as not to be peeled off from the disc substrate 30. Besides, the protective film 34 needs to cover the reflective film or the signal recording layer 33 with a thickness large enough to prevent intrusion of air or moisture into the reflective film or the signal recording layer 33.
However, if the disc substrate 30 is designed for a small-sized optical disc such that the signal recording region 32 is extended to the inner most area of the disc, the annular groove 31 of a larger width is formed at the inner side of the disc substrate 30, so that the protective film 34 at the inner most side of the disc substrate 30 can be deposited on the planar major surface of the disc substrate 30 only by a narrow width W. Consequently, the reflective film or the signal recording layer 33 cannot be covered positively by the protective layer 34 so that the reflective film or the signal recording layer 33 cannot be reliably protected against chemical attack by air or moisture.
On the other hand, the disc substrate 30 has a centering aperture 35 engaged by a centering member provided on a disc table of a disc rotating and driving unit adapted for rotating and driving the optical disc prepared from the disc substrate 30. Besides, an annular projection or boss 36 defining the centering aperture 35 is formed at a surface of the disc substrate 30 opposite to the surface on which the signal recording layer 33 is formed. The function of the projection 36 is to set the loading height of the disc on the disc table as well as to permit disc rotation in a horizontal position with respect to the center of rotation of the disc table. Consequently, the end face of the projection 36 functions as a reference plane 37 for enabling the disc to be attached to the disc table in a horizontal position with respect to the center of rotation, so that it is a planar surface for maintaining correct horizontality with respect to the center of rotation.
Meanwhile, the centering aperture 35 provided in the disc substrate 30 is bored by the punch 26 reciprocated in the mold cavity of the metal mold device. The aperture 35 is usually bored by actuating the punch immediately after the disc substrate 30 is molded by injecting the synthetic resin into the mold cavity. Above all, if the centering aperture 35 is bored while the synthetic resin injected into the mold cavity is yet in the molten state and is not completely cured, punching burrs 38 tend to be produced in the proceeding direction of the punch.
If these burrs 38 are produced on the loading reference surface 37 at the end of the projection 36 or on the loading reference surface at the inner surface of the centering aperture 35 engaged by the centering member, the centering aperture 35 is worsened in true circularity, while the reference surface 37 cannot be planar, as a result of which the optical disc produced from the disc substrate 30 cannot be loaded on the disc table with correct centering and positioning and consequently the disc cannot be maintained in the correctly horizontal state with respect to the center of rotation of the disc table to give rise to failure in recording or reproduction.